[PATCH RFC 2/2] efi: Add API for retrieving the EFI secret for cryptodisk

[Hyman Huang]

This module is designed to provide an efisecret command which
interrogates the EFI configuration table to find the location of the
confidential computing secret and tries to register the secret with
the cryptodisk.

The secret is stored in a boot allocated area, usually a page in size.
The layout of the secret injection area is a header

|GRUB_EFI_SECRET_TABLE_HEADER_GUID|len|

with entries of the form

|guid|len|data|

the guid corresponding to the disk encryption passphrase is
GRUB_EFI_DISKPASSWD_GUID and data must be a zero terminated string.
To get a high entropy string that doesn't need large numbers of
iterations, use a base64 encoding of 33 bytes of random data.

Signed-off-by: Hyman Huang <yong.huang@smartx.com>
---
 grub-core/Makefile.core.def    |   8 +++
 grub-core/disk/efi/efisecret.c | 123 +++++++++++++++++++++++++++++++++
 include/grub/efi/api.h         |  15 ++++
 3 files changed, 146 insertions(+)
 create mode 100644 grub-core/disk/efi/efisecret.c

diff --git a/grub-core/Makefile.core.def b/grub-core/Makefile.core.def
index 1571421d7..f4a65b86a 100644
--- a/grub-core/Makefile.core.def
+++ b/grub-core/Makefile.core.def
@@ -810,6 +810,14 @@ module = {
   enable = efi;
 };
 
+module = {
+  name = efisecret;
+
+  common = disk/efi/efisecret.c;
+
+  enable = efi;
+};
+
 module = {
   name = lsefimmap;
 
diff --git a/grub-core/disk/efi/efisecret.c b/grub-core/disk/efi/efisecret.c
new file mode 100644
index 000000000..a323c84bf
--- /dev/null
+++ b/grub-core/disk/efi/efisecret.c
@@ -0,0 +1,123 @@
+#include <grub/err.h>
+#include <grub/misc.h>
+#include <grub/cryptodisk.h>
+#include <grub/efi/efi.h>
+#include <grub/efi/api.h>
+#include <grub/dl.h>
+
+GRUB_MOD_LICENSE ("GPLv3+");
+
+static grub_packed_guid_t secret_guid = GRUB_EFI_SECRET_TABLE_GUID;
+static grub_packed_guid_t tableheader_guid = GRUB_EFI_SECRET_TABLE_HEADER_GUID;
+static grub_packed_guid_t diskpasswd_guid = GRUB_EFI_DISKPASSWD_GUID;
+
+/*
+ * EFI places the secret in the lower 4GB, so it uses a UINT32
+ * for the pointer which we have to transform to the correct type
+ */
+struct efi_secret {
+  grub_uint64_t base;
+  grub_uint64_t size;
+};
+
+struct secret_header {
+  grub_packed_guid_t guid;
+  grub_uint32_t len;
+};
+
+struct secret_entry {
+  grub_packed_guid_t guid;
+  grub_uint32_t len;
+  char data[0];
+};
+
+static void
+grub_efi_secret_put (const char *arg __attribute__((unused)), char **ptr)
+{
+  struct secret_entry *e = (struct secret_entry *)(*ptr - (long)&((struct 
secret_entry *)0)->data);
+
+  /* destroy the secret */
+  grub_memset (e, 0, e->len);
+  *ptr = NULL;
+}
+
+static grub_err_t
+grub_efi_secret_find (struct efi_secret *s, char **secret_ptr)
+{
+  int len;
+  struct secret_header *h;
+  struct secret_entry *e;
+  unsigned char *ptr = (unsigned char *)(unsigned long)s->base;
+
+  /* the area must be big enough for a guid and a u32 length */
+  if (s->size < sizeof (*h))
+    return grub_error (GRUB_ERR_BAD_ARGUMENT, "EFI secret area is too small");
+
+  h = (struct secret_header *)ptr;
+  if (grub_memcmp(&h->guid, &tableheader_guid, sizeof (h->guid)))
+    return grub_error (GRUB_ERR_BAD_ARGUMENT, "EFI secret area does not start 
with correct guid\n");
+  if (h->len < sizeof (*h))
+    return grub_error (GRUB_ERR_BAD_ARGUMENT, "EFI secret area is too 
small\n");
+
+  len = h->len - sizeof (*h);
+  ptr += sizeof (*h);
+
+  while (len >= (int)sizeof (*e)) {
+    e = (struct secret_entry *)ptr;
+    if (e->len < sizeof(*e) || e->len > (unsigned int)len)
+      return grub_error (GRUB_ERR_BAD_ARGUMENT, "EFI secret area is 
corrupt\n");
+
+    if (! grub_memcmp (&e->guid, &diskpasswd_guid, sizeof (e->guid))) {
+      int end = e->len - sizeof(*e);
+
+      /*
+       * the passphrase must be a zero terminated string because the
+       * password routines call grub_strlen () to find its size
+       */
+      if (e->data[end - 1] != '\0')
+       return grub_error (GRUB_ERR_BAD_ARGUMENT, "EFI secret area disk 
encryption password is not zero terminated\n");
+
+      *secret_ptr = e->data;
+      return GRUB_ERR_NONE;
+    }
+    ptr += e->len;
+    len -= e->len;
+  }
+  return grub_error (GRUB_ERR_BAD_ARGUMENT, "EFI secret aread does not contain 
disk decryption password\n");
+}
+
+static grub_err_t
+grub_efi_secret_get (const char *arg __attribute__((unused)), char **ptr)
+{
+  unsigned int i;
+
+  for (i = 0; i < grub_efi_system_table->num_table_entries; i++)
+    {
+      grub_packed_guid_t *guid =
+        &grub_efi_system_table->configuration_table[i].vendor_guid;
+
+      if (! grub_memcmp (guid, &secret_guid, sizeof (grub_packed_guid_t))) {
+       struct efi_secret *s =
+         grub_efi_system_table->configuration_table[i].vendor_table;
+
+       return grub_efi_secret_find(s, ptr);
+      }
+    }
+  return grub_error (GRUB_ERR_BAD_ARGUMENT, "No secret found in the EFI 
configuration table");
+}
+
+static struct grub_secret_entry secret = {
+  .name = "efisecret",
+  .get = grub_efi_secret_get,
+  .put = grub_efi_secret_put,
+};
+
+GRUB_MOD_INIT(efisecret)
+{
+  grub_cryptodisk_add_secret_provider (&secret);
+}
+
+GRUB_MOD_FINI(efisecret)
+{
+  grub_cryptodisk_remove_secret_provider (&secret);
+}
diff --git a/include/grub/efi/api.h b/include/grub/efi/api.h
index d44d00ad7..c41816d8b 100644
--- a/include/grub/efi/api.h
+++ b/include/grub/efi/api.h
@@ -314,6 +314,21 @@
     { 0x9a, 0x16, 0x00, 0x90, 0x27, 0x3f, 0xc1, 0x4d } \
   }
 
+#define GRUB_EFI_SECRET_TABLE_GUID \
+  { 0xadf956ad, 0xe98c, 0x484c, \
+    { 0xae, 0x11, 0xb5, 0x1c, 0x7d, 0x33, 0x64, 0x47} \
+  }
+
+#define GRUB_EFI_SECRET_TABLE_HEADER_GUID \
+  { 0x1e74f542, 0x71dd, 0x4d66, \
+    { 0x96, 0x3e, 0xef, 0x42, 0x87, 0xff, 0x17, 0x3b } \
+  }
+
+#define GRUB_EFI_DISKPASSWD_GUID \
+  { 0x736869e5, 0x84f0, 0x4973, \
+    { 0x92, 0xec, 0x06, 0x87, 0x9c, 0xe3, 0xda, 0x0b } \
+  }
+
 #define GRUB_EFI_ACPI_TABLE_GUID       \
   { 0xeb9d2d30, 0x2d88, 0x11d3, \
     { 0x9a, 0x16, 0x0, 0x90, 0x27, 0x3f, 0xc1, 0x4d } \
-- 
2.39.3